1. Field of the Invention
The present invention relates to an optical device for forming an electrostatic latent image on a photosensitive member by means of a laser beam applied to image forming apparatus such as laser printers and digital copying apparatus.
2. Description of the Related Art
In recent years, many image forming apparatus for laser printers, copying apparatus and the like constructed so as to form an electrostatic latent image by irradiating a photosensitive member with a laser beam.
An optical device of an image forming apparatus of the aforesaid type irradiates the photosensitive member with a scanning laser beam via a polygonal mirror to accomplish the exposure of the photosensitive member along the scan line.
The linearity of the scan line on the surface of the photosensitive member is reduced due to errors and the like of the optical system components such as the lenses, polygonal mirror and the like, which produces distortion or so-called bowing. In this case, the formed image is distorted, thereby reducing image quality.
Specifically when forming multicolor images by forming a plurality of electrostatic latent images corresponding to each color via a plurality of laser beams, the bowing causes color dislocation which results in markedly reduced image quality.
Optical devices of image forming apparatus designed to reduce color dislocation by providing laser beam direction switching members for directing the bowing in the same direction are well known.
Conventional optical devices for image forming apparatus are designed to reduce color dislocation by directing the bowing in the same direction, but since the generation of the bow itself is not prevented, the reduction in image quality caused by the distortion of the image cannot be suppressed. Conventional devices have a further disadvantage inasmuch as color dislocation cannot be reliably prevented when the bow curvatures are not identical.